1. Field of the Invention
This invention relates to new polyester polyols based on selected starting materials and to their use in combination with certain lacquer polyisocyanates as binders or binder components in two-component polyurethane lacquers for producing coatings with high resistance to yellowing on molded plastic parts
2. Description of the Prior Art
It has long been known to coat relatively rigid substrates such as metal or wood with two-component polyurethane lacquers based on polyesters, polyethers or polyacrylates containing hydroxyl groups and organic polyisocyanates The lacquer coatings obtained are distinguished particularly by their excellent hardness, very firm adherence and high weather resistance. The chemical basis of these lacquers and coatings is described inter alia in "Lackunstharze", Hans Wagner/Hans Friederich Sarx, Carl Hanser Verlag, Munich, pages 153 to 173, 1971.
The two-component polyurethane lacquers known in the art, however, frequently give rise to highly cross-linked lacquer coats whose elasticity and resistance to yellowing are frequently insufficient for coatings on flexible substrates, especially when exposed to shortwave UV light.
It is particularly in the motor car industry that flexible plastic parts are increasingly used for greater safety. In some countries, for example, motor vehicles are required to be equipped with bumpers which will not be damaged by impact against a solid barrier at a particular speed (e.g. 5 mph) and functional parts such as headlights, blinkers, doors and bonnets must also remain undamaged. In many countries, these and other more stringent requirements have caused metal bumpers to be discarded in the construction of motor vehicles and replaced by complete front and rear parts of highly flexible materials which fulfill the requirement for undamaged restoration of the bumpers after impact (avoiding damage in minor accidents).
Highly flexible plastics which have proved suitable for this purpose include inter alia semi-rigid elastomeric polyurethanes (e.g. Bayflex systems of Bayer AG, Leverkusen) which are produced from two-component polyurethane-forming mixtures by the reaction injection molding process in closed molds, optionally with foaming; thermoplastic polyurethanes (e.g. Desmopan of Bayer AG or Texin of Mobay Corporation, Pittsburgh, USA) produced by injection molding as well as many different types of rubbers.
The plastic parts based on these chemical materials, which have been used for some years now in the state of the art for the manufacture of motor vehicles, are relatively large and therefore to a large extent determine the appearance of the vehicle. It is therefore necessary to lacquer these parts and it is also necessary to protect the surfaces of these plastics as they are degraded by the influence of weather.
Plastic parts which are less highly elastic are also required to be covered with elastic lacquer coats to prevent mechanical damage. Thus, for example, rigid but tough thermoplasts are required to be lacquered with highly elastic, extremely resistant lacquers to ensure that mechanical damage and other external influences will not produce cracks in the lacquer film which could be propagated through the solid plastic.
Highly flexible lacquers which are perfectly satisfactory in their elasticity and their flexibility in the cold can be obtained from two-component systems known in the art, especially when certain polyhydroxyl polyesters which are synthesized predominantly from aliphatic diols and have a hydroxy functionality not significantly above 2 are used as the main polyol component. The disadvantage of these two-component polyurethane lacquers based on such polyester diols and the conventional lacquer polyisocyanates lies in the fact that the lacquer coats obtained have insufficient weather resistance and resistance to chalking so that the gloss retention of the coatings obtained from such lacquer systems is entirely unsatisfactory. Another particular disadvantage is the tendency of such lacquer systems to undergo yellowing when exposed to shortwave UV light. This applies particularly to the systems according to DE-OS 3,421,122 which in other respects substantially fulfills the requirements of elastic synthetic resin lacquers.
It was therefore an object of the present invention to provide a new two-component polyurethane lacquer system which would satisfy these special requirements and would in particular be optimally suited for lacquering elastic parts made of plastic material. The new lacquer system should in particular also satisfy the following requirements:
1. Film surfaces free from yellowing under the action of shortwave UV light: PA1 2. Low temperature elasticity: PA1 3. Good gloss retention: PA1 4. No increase in brittleness under exposure to the weather: PA1 5. Low drying temperature: PA1 6. Possibility of repair: PA1 (a) about 52 to 60 mole % of a polyol component with PA1 (b) about 40 to 48 mole % of dicarboxylic acid component, characterized in that PA1 (a) the polyol component is based on (i) about 2 to 30 mole % of at least one dihydric aliphatic alcohol which has at least two carbon atoms and is different from neopentyl glycol, (ii) about 5 to 40 mole % of at least one aliphatic alcohol which is at least trihydric and has at least 3 carbon atoms, (iii) about 20 to 60 mole % of bis-(hydroxymethyl)-cyclohexane and (iv) about 10 to 50 mole % of neopentyl glycol, and PA1 (b) the dicarboxylic acid component is based on (v) about 50 to 99 mole % of at least one aliphatic, saturated dicarboxylic acid or an anhydride of such an acid containing at least 2 carbon atoms and (vi) about 0.5 to 50 mole % of at least one aliphatic unsaturated dicarboxylic acid or its anhydride having at least 4 carbon atoms,
The lacquer film must be able to withstand irradiation with shortwave UV light (wavelengths &lt;400 nm) for a period of at least 200 hours without yellowing. PA2 The film must not break under impact even at a temperature of -40.degree. C. The mechanical properties of the plastic should as far as possible be unimpaired by the lacquering. PA2 The lacquer film should be highly durable on the plastic parts. It must not be degraded more rapidly by weather conditions than the car body. PA2 The film must not subsequently become brittle even after prolonged exposure to weather conditions, i.e. the low temperature elasticity should not be impaired by exposure to the weather. PA2 Such large plastic parts have only a limited resistance to bending under heat. The drying temperatures and times should therefore be as low as possible for this reason as well as for saving energy. PA2 The drying conditions of such a system should be suitable not only for initial lacquering but also for repair lacquering at 80.degree. C. or room temperature.
It has now surprisingly been found that these objects may be achieved by using the polyester polyols according to the invention (described in more detail below) based on selected starting materials as the polyhydroxyl component in two-component polyurethane lacquers.